In some flow systems, such as flow systems arranged under ground or offshore, it is difficult and undesirable to stop fluid flow or dismantle valve parts in order to perform inspection and/or maintenance on various valve parts of valve systems arranged in the flow system. On the other hand, it is crucial that valve systems arranged in such flow systems operate correctly and reliably, and therefore regular inspections and maintenance operations are necessary.
GB 818,798 discloses a gate valve comprising a valve member being movable by an actuating member in a direction parallel to its valve seating. The valve member is connected to the actuating member by parallel links and co-operating with stationary guides, whereby during the initial part of the opening movement and the final part of the closing movement of the actuating member, the valve member will be moved away from or towards the valve seating as the case may be, the parallel links being adapted to maintain the valve member parallel with the seating. The valve housing may be provided with a transverse slot so as to enable a damper plate to be inserted therein to seal the bore of the valve body and prevent fluid passing through the valve from escaping through the upper end of the valve body. This allows the valve elements to be removed from the valve system for inspection and/or maintenance purposes without requiring that the fluid flow through the valve is interrupted. However, the damper plate must be specially and manually introduced in the transverse slot, thereby requiring an additional step in order to provide the required sealing of the valve body. Furthermore, it is necessary to remove the valve elements from the valve system in order to perform the inspection and/or maintenance.